The microflora surrounding plants is very diverse, including bacteria, fungi, yeast, algae. Some of these microorganisms may be deleterious to plants, and are often referred to as pathogens, while others may be beneficial to plants by promoting plant growth and crop productivity. Recent advances in soil microbiology and plant biotechnology have resulted in an increased interest in the use of microbial agents in agriculture, horticulture, forestry and environmental management. In particular, a number of microorganisms known to be present in soil ecological niche, generally known as rhizosphere and rhizoplane, have received considerable attention with respect to their ability to promote plant growth. Indeed, the rhizosphere soil represents a good reservoir of microbes for the potential isolation of beneficial microbes. Plant rhizosphere can contain billions of microorganisms in one gram of soil. In theory, microbial inoculants, without human intervention, have a low survival rate and efficacy in their natural soil environment because of the insufficient colony forming units per gram of soil. Therefore, since the 1960's, a number of biofertilizers that have an increased colony inoculum potential concentration have been developed and commercialized in an attempt to reduce the need for chemical fertilizers.
In addition, research conducted in recent years has shown that microorganisms can be used as biological control agents to increase agricultural productivity and efficiency. These studies have shown that various microorganisms are able to suppress plant pathogens and/or supplement plant growth, thus offering an attractive alternative to chemical pesticides with are less favored because of their potentially negative impact on human health and environment quality.
Microorganisms which can colonize plant roots and stimulate plant growth are generally known as plant growth-promoting microbes (PGPM). In the past two decades, many PGPM species having positive influence on the growth of a wide variety of crop plants have been reported. PGPM are often universal symbionts of higher plants, and are able to enhance the adaptive potential of their hosts through a number of mechanisms, such as the fixation of molecular nitrogen, the mobilization of recalcitrant soil nutrients (e.g., iron, phosphorous, sulfur etc.), the synthesis of phytohormones and vitamins, and the decomposition of plant materials in soils which often increases soil organic matter. Also, certain microbes can facilitate plant growth by controlling microbial species pathogenic to the plant (i.e., phytopathogens). For example, some beneficial microbes can control root rot in plants by competing with fungi for space on the surface of the plant root. In other instances, competition among various microbial strains in a plant's native microflora can stimulate root growth and increase the uptake of mineral nutrients and water to enhance plant yield. Therefore, biofertilizers can be developed as products based on microorganisms that naturally live in the soil. By increasing the population of beneficial microorganisms in the soil through artificial inoculation, these soil microorganisms can boost their biological activity and, thus, supply the plants with important nutrients and beneficial factors that enhance their growth.
The inoculation of cultivated plants with PGPM is generally seen as a promising agricultural approach, for it allows pests to be controlled without using pesticides in large amounts. As environmental concerns about groundwater quality with excess fertilizer and pesticide exposure in foods grow, biological alternatives are becoming necessary. Thus, developing biological treatment compatible with fertilizers and pesticides or even reducing the amount of these chemical compounds could be a significant advancement in the agricultural industry. It has been established that stimulation of plant growth by PGPM is often closely related to the ability of the PGPM to colonize plant roots. However, relatively little attention has been given to the development of efficient selection procedures for obtaining microbial strains with high root-colonizing ability. The lack of such selection procedures slows down the study of plant-bacterial symbioses, and the deployment of PGPM in agriculture.
Therefore, there is a continuing need for the identification of new PGPM and/or testing of their compatibility with existing commercially available crop management products. Moreover, additional investigation is also needed to compare pure culture strains versus complementary mixed strains of microorganisms that form synergistic consortia. Such mixed consortia might have greater potential for consistent performance with better competitive ability under different environmental and growth conditions.